1. The swelling and bubbling of yeast when mixed with warm water and set aside in a warm location. This proves that the yeast is active. Proof is also a term that references the swelling or rising of dough just before baking. 2. A term used to indicate the amount of alcohol in liquor. The poof is exactly twice the amount of the percentage of alcohol contained in the liquor. "90 proof" would contain 45 percent alcohol.